Technical Abstract:
Rosette disease (incited by Cercosporella rubi is severe on erect blackberries grown in the southeastern U.S. and may reduce yields by more than 50% by the 4th year of production. The fungal pathogen causes distorted growth on the host which are first evident in early spring on floricanes when numerous leafy shoots develop from each infected vegetative bud. These shoots (rosettes) are smaller than normal with pale green foliage that later turns bronze. Infected flower buds are more elongated, larger, coarser,and redder than uninfected buds. Petals of infected flowers are wrinkled and twisted with a pink tint. The fungal mycelium covers the pistils and anthers and produces conidia that can infect young emerging primocanes. Chemical control of this disease has been erratic. The objectives of this study were to identify fungicides that control rosette and to determine the optimum application schedule. Benomyl and Bordeaux mixture were the most effective of 11 fungicides tested, but their efficacy was dependent on applications starting at bloom and continuing until all infected flowers died. DCNA, myclobutanil, ferbam, metalaxyl, and propiconazole gave limited control, while triadimefon, vinclozolin and iprodione were ineffective. These fungicide studies and field observations indicated that the infection period for rosette extends throughout the time that C. rubi is sporulating on infected blackberry flowers which extends from mid-March to mid-July in the Southern U.S.